FIG. 1 depicts a communication environment 10. The communication environment 10 may be a cell in a cellular telephone network or an area covered by a cordless telephone system. As shown, a number of remote, portable transceivers 12, 14 and 16 are provided. Each of the portable transceivers 12, 14 and 16 can maintain a separate duplex communication with a base station 18. The portable transceivers 12, 14 and 16 may be cellular telephones and the base station 18 may be a cell base station. Alternatively, the portable transceivers 12, 14 and 16 may be cordless handsets of a cordless telephone and the base station may be the telephone line terminal of the cordless telephone.
In either case, the portable transceivers 12, 14 and 16 are free to move about the environment 10. When desired, a user of a portable transceiver, e.g., the transceiver 12, activates the transceiver 12 and causes it to initiate communication with the base station 18. Once communication is established between the base station 18 and the transceiver 12, the user may obtain access to the line communication network 20 via the portable transceiver 12, the environment 10, the base station 18 and the line network 20.
Generally speaking, it is not desirable to maintain communication between the portable transceivers 12, 14 and 16 and the base station 18 unless the user is operating the portable transceiver 12, 14 or 16 and remains in the environment 10. For instance, if the portable transceiver 12 is communicating with the base station 18, and moves outside of the environment 10, it may be desirable to disengage the communication link between the portable transceiver 12 and the base station 18. Instead, it may be desirable to engage communication between the portable transceiver 12 and a base station at the new environment into which the portable transceiver 12 has moved. Such is the case where a cellular telephone 12 moves from one cell 10 to another. Alternatively, if the user is not utilizing the portable transceiver 12, it is usually desirable to disengage communication between the portable transceiver 12 and the base station 18 to conserve the battery power of the portable transceiver 12. Thus, the number of portable transceivers 12, 14 and 16 which communicate with the base station 18 is dynamic, i.e., varying over time.
With these communication considerations in mind, the manner in which communication is achieved between the portable transceivers 12, 14 and 16 and the base station 18 is now discussed. The portable transceivers 12, 14 and 16 and the base station 18 communicate with each other by transmitting a bitstream which is organized into a frame structure. FIG. 2 shows an illustrative frame 30 according to the Digital European Cordless Telecommunications (DECT) standard. However, the discussion is general enough to apply to the pan-European digital cellular telephone system (GSM), the second generation cordless telephone (CT2) and the U.S. digital cellular system (IS-54). The frame 30 comprises 11,520 consecutive bits of the bitstream. Illustratively, the bit rate of the bit stream is 1152 kbits/sec. Thus, each frame 30 has a duration of 0.01 seconds.
As shown, each frame 30 is divided into 24 time slots 31-0, 31-1, 31-2, . . . , 31-23. Each time slot, e.g., the time slot 31-0, is allocated 480 bit positions, although, as discussed below, bits are not always transmitted during these respective bit positions of a slot.
A "full" time slot 31-0, i.e., a time slot allocated for, and currently utilized in, communication is also shown in greater detail in FIG. 2. Each full time slot 31-0 begins with a 16-bit preamble followed by a predetermined 16-bit synchronization word (referred to herein as a sync word) in the bits designated "S". Next, 48-bit control data is transmitted in the bits designated "A". Such control data may be used to instruct the portable transceivers 12, 14 or 16 or the base station 18 in performing some communication procedure or call set-up step. Following the control data is a 16-bit cyclical redundancy code for the control data, designated "A-CRC." The A-CRC bits are formed from the preceding A control data bits. Following the A bits and the A-CRC are 320 bits of user data designated "B." The B bits contain the actually transmitted data such as data representative of the user's speech. Immediately following the B bits is a 4-bit cyclical redundancy code for the B bits referred to as X-CRC bits (not shown). Next is a copy of the X-CRC bits designated "Z." Finally, 56 guard bits designated "G" are provided to allow for timing uncertainty and frequency switching.
In operation, each of the time slots 31-1, 31-2, . . . , 31-23 is allocated to one half of a duplex communication channel, i.e., for communicating in one direction between the base station 18 and one of the portable transceivers 12, 14 or 16. As shown in FIG. 2, half of the time slots 31-0, 31-1, . . . , 31-11 are allocated to the base station 18 for transmitting information to the portable transceivers 12, 14 and 16, while the other half of the time slots 31-12, 31-13, . . . , 31-23 are allocated to the portable transceivers 12, 14 and 16 for transmitting information to the base station 18. With 24 total time slots, up to 12 duplex communication channels can be supported simultaneously. The allocation of channels and corresponding time slots is not fixed, but rather varies depending on the communication requirements of the system. That is, as the portable transceivers 12, 14 and 16 require communication service, the base station 18 allocates the available channels and slots. Likewise, the base station de-allocates channels which are no longer necessary to maintain communications thereby making those channels available for allocation. Such a time division multiplexing scheme is referred to as time division multiple access (TDMA).
As mentioned above, FIG. 2 depicts the situation where each time slot 31-0, 31-1, . . . , 31-23 is full, i.e., bits are transmitted in each bit position of each time slot. This is not always the case. For instance, not every time slot which is assignable to portable transceivers need be allocated at any one time. In such cases, the bit positions of the non-allocated slot will be empty. However, the base station 18 always transmits "S" and "A" bits for one certain time slot, even if no other slot is allocated to a portable transceiver. Thus, at least one time slot with "S" and "A" bits may be detected by another device during each frame 30.
In such a frame structured communication scheme, it is important that each portable transceiver 12, 14 and 16 be synchronized with the frames (i.e., in relation to the base station 18) initially, when it is desired to initiate a communication, and to maintain such synchronization during communication. The former is referred to herein as "frame timing acquisition" and the latter as "frame timing maintenance." In the GSM and IS-54 systems, one channel is always active for purposes of distributing system information from the base station 18 to the portable transceivers 12, 14 and 16. In such systems, frame timing acquisition, and later call-setup, may be accomplished using this continuously active channel.
However, some systems, such as DECT and CT2 do not have such a continuously active channel or a channel dedicated for this purpose. In such systems, frame timing acquisition is achieved as illustrated in the flow chart of FIG. 3. In a first step 52, the portable transceiver enters the acquisition period wherein certain parameters are initialized. Next, the portable transceiver executes step 54, wherein the portable transceiver performs a bit-by-bit search on the transmitted bitstream for a sequence of bits corresponding to the aforementioned sync word of the S bits. Each time a bit is received, the transceiver executes step 56 to determine if the sequence of the last 16 received bits forms a sync word. If not, the portable transceiver returns to step 54. If a sync word is detected, the transceiver executes step 58 wherein a counter k is set to 0. Next, in step 60, the portable transceiver determines, based on the last received sync word, where the next sync word of the next slot (one frame later, assuming a worst case scenario wherein only the base station is transmitting a single time slot per frame with S and A bits) should occur. That is, the transceiver sets its remote, internal clock to be synchronized with the occurrence of the detected sync word. Furthermore, the portable transceiver monitors the bitstream for such a subsequent occurring slot (one frame later) and determines if another sync word is present at the appropriate bit positions of the bitstream. If not, then the portable transceiver had originally detected some other bit-pattern similar to the sync word (e.g., a subsequence of the B bits which was identical to the sync word) but not the sync word itself. Thus, if no subsequent sync word is found, the portable transceiver returns to step 52 to try to identify an initial sync word. However, if a sync word is detected at the expected location in step 62, then the portable transceiver proceeds to step 64 wherein the counter k is incremented by 1. Next, in step 66, the portable transceiver determines if a predetermined threshold number K of consecutive sync words has been detected (i.e., k.gtoreq.K). If not, then the portable transceiver returns to step 60. However, if k is greater than or equal to K then the transceiver proceeds to step 68 wherein the slots are monitored to determine which of the slots is the first slot of the frame (and therefore adjacent to the frame start boundary). Once this is determined, the remote clock at the transceiver is considered synchronized to the frames of the bitstream (and therefore the local clock at the base station).
To summarize, the portable transceiver monitors the bitstream, bit-by-bit to attempt to identify an initial sync word. This may in fact be a sync word or a subsequence of the bitstream which is similar to the sync word. If the portable transceiver misidentifies an arbitrary non-sync word portion of the bitstream as an initial sync word, a "false alarm" is said to occur. To ensure that the sync word was in fact received, the portable transceiver attempts to identify K more sync words at an appropriate location in the bitstream, assuming that the initially received pattern was in fact a sync word. If K such sync words are received, then the initially detected subsequence can be presumed to be a sync word of the bitstream and the portable transceiver is synchronized with the time slots of the bitstream. The portable transceiver can therefore easily acquire the frame by determining which time slot is the first slot.
To complicate this conventional procedure, there is a probability that any given received bit may be misidentified, i.e., an originally transmitted logic `0` bit is misidentified by the portable transceiver as a logic `1` bit or an originally transmitted logic `1` bit is misidentified by the portable transceiver as a logic `0` bit. This is referred to as a bit error. (Bit errors occur as a result of a variety of random well known reasons such as thermal drift of bit clocks at the bases station, the portable transceiver, or both, interference noise in the environment, the dynamic movement of physical objects in the environment, etc.) The probability for a bit error can be as high as p=10.sup.-3 in the communication system 10 of FIG. 1. Such a high probability is tolerable for voice communication. However, if a bit error occurs in a sync word, the likelihood that the sync word may not be detected increases. A failure to detect a sync word, as a result of a bit error therein, is referred to herein as a "miss." To aid in detecting sync words (in steps 54 and 60 of FIG. 3) despite the potential presence of bit errors therein, the portable transceiver does not require matching an exact subsequence of bits of the bitstream to the predetermined copy of the sync word generated therein. Rather, the portable transceiver tolerates a correlation threshold number E of unmatched bits between the received sequence of bits and the predetermined sync word pattern generated in the portable transceiver. If the length of the sync word is N then the probability Q of a miss in detecting the sync word is: ##EQU1## The probability F of a false alarm is given by: ##EQU2## Table 1 represents illustrative values for Q and F during acquisition for different thresholds E.
TABLE 1 ______________________________________ Corr. Thresh. E Miss Rate False Alarm Rate ______________________________________ E = 0 1.59 .times. 10.sup.-2 1.526 .times. 10.sup.-5 E = 1 1.19 .times. 10.sup.-4 2.59 .times. 10.sup.-6 E = 2 5.56 .times. 10.sup.-7 2.09 .times. 10.sup.-3 E = 3 3.37 .times. 10.sup.-9 1.06 .times. 10.sup.-2 ______________________________________
As can be seen, the miss rate decreases with increasing E but the false alarm rate increases with increasing E.
In the case of ordinary time division multiplexing (TDM) transmission via line networks, a prior art frame maintenance method and system have been proposed in U.S. Pat. No. 4,316,284. The system disclosed in this patent is specifically designed for use with the well known DS1 super frame structure which is reproduced in FIG. 4. As shown in FIG. 4, a DS1 super frame consists of 24 frames wherein each frame includes 193 bits. Each frame is separated from a neighboring frame by a framing bit F, subframe signalling information bit M or a cyclical redundancy code bit C. The F-bits, when appended together form a predetermined framing pattern. The C bits, when appended together form a cyclical redundancy code which can be used to verify the accuracy of the framing pattern in an extended super frame following the current extended super frame in which the C bits are contained.
According to this reference, a framing bit pattern is locally generated. In response to a locally generated framing clock, each bit of the locally generated framing pattern is compared to a received bit of the extended super frame of the bitstream, which received bit is believed to be a corresponding framing bit. Whenever two locally generated framing bits out of four do not match the received bits of the bitstream to which they are compared, a loss of frame signal is generated. In addition, a cyclical redundancy code is generated on the bits of a currently received super frame of the received bitstream. In response to a locally generated CRC clock, each bit of the locally generated cyclical redundancy code is compared to a received bit of the extended super frame of the bit stream, which received bit is believed to be a corresponding C bit. Whenever, a predetermined number of CRC errors are detected by means of this comparison, a loss of CRC signal is generated. Note, however, that the bits of a cyclical redundancy code generated in a currently received extended super frame is compared to the C bits of the very next received extended super frame. The generation of either the loss of frame signal or the loss of CRC signal indicates that re-synchronization with the bitstream is necessary. To determine when to initiate a re-synchronization, every bit is tested (concurrently with the above framing and CRC checks) to see if it is a framing bit by appending it to the 772.sup.nd, 1514.sup.th and 2268.sup.th received bits preceding the tested bit. The framing pattern thus formed is inputted to a logic circuit to determine if it contains a potentially valid framing pattern of a set of potentially valid framing patterns. If so, a signal is generated to indicate that a valid framing pattern was received. Therefore, in response to the loss of frame or the loss of CRC signals, a frame re-synchronization is initiated when the next potentially valid framing pattern signal is generated.
The prior art method for the DECT and CT2 system are disadvantageous because they require a relatively long time to acquire the frame timing. The long time requirement arises because K+1 consecutive sync words must be identified (i.e., correlated with a predetermined sync word) to acquire the frame time. This time lag is exacerbated by the bit error problem, which introduces the possibilities of misses. As mentioned above, tolerance thresholds can alleviate the miss problem at the expense of the false alarm problem. Note that even in the absence of bit errors and false alarms, if K is set equal to 1, but only the base station is transmitting S and A bits during one time slot each frame (no active use of channels) then at least two frame times are required to acquire frame timing synchronization.
The method and system disclosed in U.S. Pat. No. 4,316,284 are adequate for frame timing maintenance in a receiver which receives a continuously transmitted bitstream from a transmitter in a TDM system. However, this solution is not adequate for the TDMA system described above. First, by definition, the bitstream in the TDMA system does not necessarily always include a continuous sequence of bits. Rather, some bit positions of non-allocated slots will be empty. Second, the bits of the sync word and cyclical redundancy code are not uniformly dispersed over the signal in the above-described TDMA system. Rather, in the TDMA system, the bits of the sync word and cyclical redundancy code are transmitted as words (i.e., aggregated into uninterrupted subsequences) and in bursts.
It is also worthy to point out that U.S. Pat. No. 4,316,284 is directed to the easier problem of frame timing maintenance and not the more difficult problem of frame timing acquisition. That is, the miss and false alarm problem are not as serious in frame timing maintenance as they are in frame timing acquisition. In frame timing maintenance, the probability of a miss is the same as shown in equation (1). However, the probability of a false alarm F.sub.w, in a window of length w, is given by: ##EQU3## Table 2 shows illustrative values for Q and F.sub.w during maintenance for different thresholds E.
TABLE 2 ______________________________________ Corr. Thresh. E Miss Rate False Alarm Rate ______________________________________ E = 0 1.59 .times. 10.sup.-2 9.92 .times. 10.sup.-25 E = 1 1.19 .times. 10.sup.-4 7.93 .times. 10.sup.-21 E = 2 5.56 .times. 10.sup.-7 2.77 .times. 10.sup.-17 E = 3 3.37 .times. 10.sup.-9 5.54 .times. 10.sup.-14 ______________________________________
In comparing tables 1 and 2, it can be seen, even with a correlation threshold of E=3, the false alarm rate in maintenance is far lower than the false alarm rate in acquisition with a correlation threshold of E=0.
Furthermore, it should be noted that the cyclical redundancy code bits contained in a current DS1 super frame are for checking the next extended super frame. Therefore, in the case of frame time acquisition, at least two extended super frame times are necessary to acquire the frame timing. Such an enormous time lag is typically greater than the time lag of the conventional DECT system.
It is therefore an object to overcome the disadvantages of the prior art.